Because the conversion is taken from a point in-line with where the try is scored, at a distance chosen by the kicker. The important point to note is that a try is not scored "when the player enter in the in-goal area": the try is scored only when the ball is grounded with control in the in-goal area. The kick is obviously much easier if it's done from in front of the posts rather than far out by the touchline, so players will attempt to ground the ball as close to the posts as possible in order to make the conversion as easy as possible.
The actual wording is different in union and league; the relevant laws are 9.B.1(b) for union:
The [conversion] kick is taken on a line through the place where the try was scored in the field of play.
and 6.8 for league:
A kick at goal after a try may be taken from any point on an imaginary line drawn parallel to the touch line in the field of play and through the point where the try was awarded.